All language subtitles for Wonders of Life 5of5 Home_Subtitles01.ENG

PROFESSOR BRIAN COX:
Ihis creature is a wonder of nature.

Its biology is hard-wired
to the heavens.

It has exguisitely sensitive eye

that locks onto the Sun

and allows it to navigate its way
across the face of the planet.

In a sense, it has
an instinctive understandıing

of its place in the solar system.

A tiny insect brain

Joined to the movements
of the Sun and the planets.

Ihis connection steers the monarch
and millions of its brethren

as they make
öne of the longest migrations

of any butterfly species.

They're heading these trees,
known locally as the oyamel

or sacred furs.

And some of the butterflies began their
journey over 4,000 kilometres away,

that's 2,500 miles,

up here in the north-eastern
United States and Canada.

And over the autumn and the winter

they've migrated south,
across the United States,

and arrived here, in Central Mexico.

Incredibly, no butterfly
has ever learnt this route.

It can't have, because İit takes
at least three generations

to make the round trip.

Instead, the homing instinct is carried
on a river of genetic information

that flows through each butterfly.

The allure of this place
to the butterflies,

this sense of belonging,

is a deep feeling we all share.

We even have a word for it.

Home.

Every living thing that we know to exist
is found on this one rock.

So what is it about our planet

that makes it such a rich,
colourful, living world?

I want to show you why our world
is the only habitable planet we know of

anywhere in the universe.

And the answer depends on the presence
of a handful of precious ingredients

that make our world a home.

(EBAGLES SCREAMING)

ASTRONAUT (ON RADIO): / tfhe begğinning,
God created the heaven and the Farth.

And the Earth was without form and void.

And darkness was upon the face
of the deep.

Home is such a...an evocative word.

I mean, it will mean something to you.

The place you went to school,
the place you lived,

the place where your kids
had their first Christmas.

But in a scientific sense,
what does it mean?

İt means that the ingredients
are there for you to live.

An atmosphere, food, water.

You need the temperature to be right.

Home is the place
that has the things you need

for your biology and chemistry to work.

It's no less evocative for that.

-(MAN GRUNTS)
-(HORSE NEIGHING)

(COW MOOING)

This is Mexico,

a country rich in the ingredients
that set our world apart.

İIt's not a bad place to come,

because with that 126 of the
land surface area of our planet,

it's home to 1296 of the species.

There are 26,000 plant species here.

There are 700 species of reptiles

and 400 species of mammals.

İt's also been home to some
of the world's great civilisations.

The Maya built their temples out there
in the forest here

for thousands and thousands of years.

(MAN SINGING IN SPANISH)

Mexico is bursting with life.

And if you know where to look,

hidden inside these creatures

are clues that tell how this planet
became their home.

First stop is in the south-east
of the country,

an area covered in thick jungle.

The Yucatan's a strip
of essentially pure limestone

that separates the Caribbean
from the Gulf of Mexico,

and it's got all the ingredients
you might think you need

for a rich and diverse ecosystem.

The tropical sun warms the forest,

delivering precious energy
to each and every leaf.

Öxygen escapes from the
plants and trees,

which is breathed in
by the forest animals.

And, where they can,

each of them draws deeply
from the regiron's hidden water supply.

But there are some
of the ingredients you need

to grow this tropical forest
that are far more important than others.

You might think that this place
would be awash with water.

And it does rain a lot
and it's incredibly humid.

Bukt, actually, there are no surface
rivers at all on the Yucatan Peninsula

because the water just seeps
into the porous limestone.

That's where these things come in.

These are cenotes,

they're caverns dissolved
out of the limestone by the rain.

And they collect water.

They play a vital role in the ecosystem.

I mean, the forest changes
when you get around a cenote.

-Just listen to that.
-(FROGS CROAKING)

Those are frogs,

and you don't hear those frogs
anywhere else in the forest,

Just around the cenotes.

The cenotes are flooded caves

that have been cut off
from the outside world

for thousands of years.

Lilies, troglodytic fish,
even the occasional turtle,

all thrive around the openings
of these freshwater welils.

As I head deeper into the cave,

the temperature drops
and the light fades.

Öne by one,
the ingredients I depend upon

begin to disappear.

Yet even here,
far from the soil and air,

strangely-coloured algae
still find a home in the water.

If there's öone thing that unites
every form oflife in this cenote,

in fact, every form of life
out there in the forest,

in fact, every form of life
we've ever discovered

anywhere on planet EFarth,

is that it has to be wet.

Only on our home does water run freely

between the skies, oceans, rivers
and on, into every living thing.

(SPEAKING IN SPANISH)

To understand why life and water
are so intertwined,

we need to look a little deeper

into öone of the strangest
substances we know.

(SPEAKING IN SPANISH)

Now, I may be
a bit of a middle-aged academic,

but I can still do the odd experiment
every now and again.

So what I'm doing is I'm charging up
this Perspex rod.

So, giving it an electric charge
by rubbing it on a fleece.

Now, watch what happens

when I put the rod
next to a stream of water.

You see that?

Look at that.

The electrical field, the electric
charge, is bending the water towards İt.

Now, the reason for that,

the reason that water behaves
in that way

when it's passing through
an electric field,

is exactly the same reason
that it is vital for all life on Earth.

Water is polar molecule,

which means it responds
to electric charge.

Its polarity comes about
because of the structure

of water molecules themselves.

Now, water is H20,

two hydrogens and one oxygen atom
bound together.

So, two hydrogen atoms approach oxygen.

Oxygen's got a cloud
of eight electrons around İt.

So when the hydrogens come in,

then what happens is the electrons
get dragged over here,

around the oxygen.

So you end up
with the electron cloud around here,

and, to some extent,

pretty isolated, positively-charged
protons out here.

So you get a net
positive charge over here,

and the electron cloud
with its negative charge over here.

So you get what's called
a polar molecule.

And that's why when you bring
a charged Perspex rod

close to water molecules,
they bend towards it.

Water's polar nature means

that, although its molecules are simple,

together they form a subtle,
endlessliy-complex ligurd.

A home in which
öne tiny creature thrives.

There he is. Look at that.

That is a pond skater,

a predator that floats
on the surface of the water

and actually uses
the surface of the water

to sense its prey.

Pond skaters are vicious predators

that live for most of their lives
on the surface.

Tiny hairs on their legs
provide a large area

that spreads their weight.

Their middle legs thrust them forward.

Hind legs are employed to steer.

They're so well adapted to life
in this flat world

that they even sense
their sexual partners

through tiny vibrations
in the water's surface.

The reason it can do that

is the result of a complex interaction

between adaptions in the animal itself

and the physics, the chemistry,
of the surface water.

Water molecules are polar,

and that means
that water molecules themselves

can bond together.

So you can get hydrogen,
with its slight positive charge

getting close to the oxygen
of another water molecule,

with its slight negative charge,

and bonding to İt.

And you can build up duite large,
in fact, very large structures

in liguid water.

This is what gives water
its unigue ability

to form a surface habitat
for the pond skaters.

Clumps of H20 stick together,

keeping the surface under tension,

forming a chorus of water molecules,

alljoined together by hydrogen bonds.

Then a pond skater comes along

and it puts its legs, or its

dangly things into the water

and pushes it down,

bends the surface of the water.

Now, the water doesn't like that
because a bend in the water

is increasing its surface area,

it's increasing its energy,

it's making it harder
for all the molecules

to bond together
with the hydrogen bonds.

So they try to push back.

They exert a force on the
pond skater's leg

because they want to bond
as much as they can.

And that's how pond skaters
stay on the surface of the water.

Hydrogen bonds do far more

than just give the pond skaters
a place to live.

They're fundamental to all life.

I've heard it said

that we won't truly understand biology
until we understand water.

These are very thin tubes of glass.

They're about, what,
a millimetre in diameter.

Ifl dip one
into the surface of this river,

you see that the water just
climbs up the tube.

İt pulls itself up, guite literally,

against the force of gravity.

Now, in trees there are tubes

which are about
half the diameter of this,

perhaps about half a millimetre
or even less.

And they are called xylem.

And they allow the tree to lift water
up through the root system,

because the water molecules
strongly attract each other

and are strongly attracted
to the sides of the tubes.

So when you look at trees like that,

which are very high,
and you ask yourself the guestion,

"How do they get the water up from
their roots to the top of the tree?"

A big part of that is capillary action,

which is down
to the polar nature of water.

Öne of water's most important gualities

is its ability to dissolve and carry

all manner of substances
around the living world.

Because its molecules are very small
and polar,

water İs a tremendously
effective solvent.

Those molecules can get
in amongst other substances,

salts and sugars, for example,

and disperse them, if you like,
in that sea of hydrogen bonds.

Within every one of us,

Water is constantiy flowing
around each and every celi.

Blood plasma is over 9096 water,

and in it are dissolved
everything I need to live.

Oxygen, the nutrients
from food, everything,

distributed around my body
in rivers of water.

We live on a beautiful, blue
anomaly of a world.

The only planet we know

With a surface drenched in liguid water.

The story of how each drop ended up here
has been hard to fathom.

Iargely,
because it happened so long ago,

there's very little direct evidence.

But back in the Yucatan jungle,

clues to how it turned up
can still be found.

Every civilisation on the Yucatan,

be it the modern Mexicans or the Mayans,

had to get their water
from those deep wells,

the cenotes.

And I've got a map,
a completely unbiased map

of the largest cenote. İt's here,

which I'm going to overlay
on the Yucatan.

Look at that. They lie in a perfect arc,

centred around a very particular village

which is there.
And it's called Chicxulub.

Now, to a geologist,

there are very few natural events

that can create a structure,

such a perfect arc as that.

All the evidence points
to just one explanation.

You're looking at what's left

of a gigantic asteroid strike.

Öne that wiped out three guarters
of all plant and animal species

when it hit the Farth
65 million years ago.

You may think that impacts from space
are a thing of the past,

a thing that only happened to dinosaurs.

But that's not true either.

About 55 million kilograms of rock
hits the Earth every year,

and around 296 of that is water.

This hints that at least some
of Farth's water arrived from space.

Iate in 2010,

these glimpses of Comet Hartley 2
arrived back on Farth.

They were sent
by NASA's Deep Impact probe.

From its surface,
dust and ice spray into space.

Analysis of this water

found it had
a very similar mixture of isotopes

to the water in our own öoceans.

Ihis was first firm evidence

that icy comets must have contributed
to the formation of our world's öoceans.

Farth began life as a molten hell.

Its internal heat
drove off any trace of moisture.

But, soon, the planet cooled

-and the first clouds grew.
-(THUNDER RUMBLING)

Then, 4.2 billion years ago,

a deluge, the like of which

the solar system had never seen
before or since, rained down.

And, ağain,
thanks to those hydrogen bonds,

Water's borling porint is high enough

to have allowed it to remain
on the surface of the Farth

to the present day.

So from duite early in its history,

our home has been able to hang on
to this most vital of ingredients.

But to trace the origin
of the next ingredient

you have to look beyond our planet.

To our nearest star.

And the rays of light it sends our way.

This is the train
from Los Mochis to Chihuahua

which inexplicably leaves
at 6:00 a.m. in the morning.

The local name for this area
in all the gulde books

is "The Land of Turtles".

Beautifully romantic name for this place
on the Sea of Cortez.

But, actually, we just found out
that it's probably more likely

to have been called
"The Land of Spinach-type Vegetables".

So we're going from "The Land
of Spinach-type Vegetables"

to Chihuahua,

which is "The Land of Very Small Dogs".

One of the great railway
Journeys of the world.

(TRAIN WHISTLE BLOWING)

Almost all life depends on the energy

that the Sun sends our way.

But the Sun is a far
from benevolent companion,

because its radiant ralin
can be as dangerous

as it is nourishing.

We're still round about sea level now,

and the Sun is guite low in the sky.

It's about 7:00 a.m.,
so it's not been up long.

And I'm going to measure
the amount of UV radiation

falling on every sguare centimetre
with this,

a digital ultraviolet radiometer.

At the moment,

it says there's about 22 microwatts
per sguare centimetre

falling on my skin.

But as we climb in altitude,

then that UVB light

is gonna have to travel through
less and less of the atmosphere,

so less of it's going to be absorbed.

And, sure enougğh, as the miles pass by

and we head
into the mountainous interior,

the meter readings start to go up.

(TRAIN WHISTLE BLOWING)

Now it's about 10:00 in the morning,

so the Sun's significantly
higher in the sky.

The train's also climbed
guite a bit in altitude.

So now,

we're getting nearly 250 microwatts
per sguare centimetre.

So that's a factor of 10 higher,

and that's just because the UVB
has had significantly less atmosphere

to travel through from the top
of the Earth's atmosphere down to me.

That's more than enougğh to burn
unprotected skin in just a few minutes.

And that's because
what arrives from the Sun

is far more than just the stuff
we can see.

Beyond the visible,

in the higher energy
part of the spectrum,

there's ultraviolet light,
particularly UVB,

which does get through
the Earth's atmosphere

and gets to the surface.

Now, UVB can be beneficial to life.

We use it to produce
vitamin D, for example.

But because it's higher energy,

it can also be extremely damaging.

It can damage DNA, it can burn our skin
as well as give us a suntan.

And, of course, ultimately,
it can give us skin cancer.

(TRAIN WHISTLE BLOWING)

Now, if ultraviolet light is a problem
for life on Earth to deal with today,

then the physicist might raise

an interesting problem
for the biologist.

Because we know
that three and half billion years ago,

when life on Earth began,

although the Sun was much dimmer

in the visible part of the spectrum,

it was significantly brighter
in the ultraviolet.

The young Sun seems like a paradox.

İt was fainter to the eye,

perhaps 3096 less bright
than the Sun we enjoy today,

yet rich in deadiy ultraviolet.

Inside, the core was spinning
much faster,

which created more
electromagnetic heating

of the plasma on its surface.

And this plasma emitted more energy,
not in the lower visible freguencies,

but in the higher freguencies.

Like X-rays.

And ultraviolet.

İt seems as if just as life
Was getting settled on its wet home,

the faint young Sun was making it tough
to survive near the surface.

This is the top of Copper Canyon,

so the summit of the railway journey.
İt's about 2,200 metres,

which is about, what, somewhere
between 7-8, 000 feet.

So I'll take a UV reading of the Sun.

Yeah, it's actually reading
about 260 now.

Now, if you remember,
at midday, down at sea level

we were getting readings around 260.

So although the Sun
has dropped in the sky,

so the sunlight and the UV
are coming through much more atmosphere,

that's been compensated for
by the thinness of the air up here.

I'm getting more UV now

than I would have been
at the same time of day at sea level.

It's hard to be sure,

but we think that
it's these kinds of radiation levels

that early life had to deal with.

Because back then
the Sun's ultraviolet output

Was significantly stronger.

So I think it is fair to say that that
could have posed a significant threat

to the development
of early life on Earth.

(HORSE NEIGHS)

Today, life has painted
the surface of our home

in all the colours of the rainbow.

From greens to blues, reds to yellows,

oranges and violets.

And the origin of all life's hues
can be traced back

to the way it interacts with sunliğht.

I'm a particle physicist,
so I'm allowed to think of everything

in terms of the interactions
of particles.

So I would picture the
light from the Sun as being,

really, a rain of particles,
photons they're called,

particles of light
of different energies,

raining down
on the surface of the Earth.

The blue ones
are the highest energy photons.

The red ones
are the lowest energy photons.

And all the colours of the rainbow
in the middle

are just simply photons
with different energies.

(SPEAKING IN SPANISH)

Ah, thank you.

Wow.

So, for this, a chilli salsa,
which I see as red,

then there are
pigment molecules in there

that are absorbing the blue photons,
the blue light from the Sun.

The red ones it doesn't interact with,
and so they bounce back into my eye

and that's why I see İt as red.

Same with the green chilli.
But in this case,

the red photons are interacting,
they're doing something,

they're talking to the pigments in here,

and what I'm seeing
are the green photons

and some of the blue photons
coming into my eye,

mixing up and allowing me
to see that as green.

(DOG WHIMPERS)

Pigmenits bring colour to the world.

A planet painted by genes,

honed by billions of years of evolution.

Some colours warn of danger...

This stuff's on fire, I tell you.

Or attract pollinators.

Pigments are öone of the ways
that life has evolved

to take on the Sun's
powerful ultraviolet light.

This little guy
is called a bombardier beetle.

If I just grab him...

His name comes
from his unigue defence mechanism.

He produces two chemicals,

oöne of them you might have heard of,
hydrogen peroxide.

And the other one
is something called hydroguinone.

And when you scare him,

both those chemicals are injected
into a little chamber in his body.

Which raises the temperature
to the boiling point of water,

and increases the pressure,

sguirting a hot, noxious chemical
Ousvt of its rear.

What a clever way to defend yourself.

But this is just öone of the ways
this character uses chemistry

to increase its chances of survival.

Now, the bombardier beetle and me

and, in fact, everything living thing
you can see,

are exposed to the same threat here
on the high plains of Mexico.

İt's the high-eneray,
ultraviolet photons

raining down on this landscape
from the Sun.

If they hit DNA,
in my skin, for example,

then they'll damage the DNA.

So that's got to be prevented.

Me and my friend the beetle

have both reached the same solution.

You see, the beetle is brown and black.

My skin, oönce exposed to the Sun,
is going brown,

because I'm producing a pigment
called melanin.

And so is the beetle.

Now, melanin is a very simple molecule,

it's just a ring of carbon atoms
with a few extra bits bolted on.

But its sea of electrons behaves
in a very specific way.

When a high-energy
ultraviolet photon from the Sun

hits one of those electrons,

it very, very gulckly
dissipates that energy.

That potentially threatening photon
has been absorbed

and all its energy
has been dissipated away as heat.

Melanin is so efficient that over 99.996

of the harmful
ultraviolet radiation is absorbed.

So melanin is protecting both my skin

and my friend the bombardier beetle

from the potentially harmful effects
of the Sun.

(TRAIN WHISTLE SOUNDS)

From the starkt,
life had to evolve strategies

for coping with the energetic young Sun.

Now, life is nothing if not resourceful,

and pigments are the way
that living things interact

with the radiation from the Sun.

So why just use them
to dissipate energy, to protect?

Why not use them to harness the energy
for its own ends?

Well, that's exactly what life did.

And, in doing so,
it transformed our planet

by introducing
a Wonderful new ingredient.

Farth has an atmosphere unlike any other
planet we know of in the universe.

Only in the air in our world
do fires burn.

Only in our world
has a gas been released

which allowed complex life to evolve.

What makes our home unigue

is its oxygen-rich atmosphere.

Deep in a cave in the hills of Tabasco,

you can find a hint of what a living
planet without oxygen miğht be like.

(WATER FLOWING)

This is one of the more
unigue environments on our planet.

This cave is full of sulphur.
You can see İit in the water,

you can see that milky colour in the
lake that's flowing through the cave.

That's dissolved sulphur.

And it's coming
from hydrogen sulphide gas,

the source of which
is actually not entirely known.

The hydrogen sulphide is toxic to me.

And it has another, rather alarming
effect on this hellhole.

İt's a bad-smelling gas, but it's also
the gas that drives the oxygen out.

So, certainly as you go on
into the cave system,

you get less and less oxygen.

So, in a sense, some of the chemistry
and the biochemistry

that takes place in the dark
in these cave systems

could be very similar
to the chemistry and biochemistry

that occurred when our planet
was very young.

For the first half of its history,

Farth was without oxygen
in its atmosphere.

Bukt, incredibiy,
in this echo of the past,

which I can only visit
for a few minutes,

there are forms of life
that are completely at home.

Wow, look at that.

There they are,

cities of sulphur-eating bacteria
living off the hydrogen sulphide gas.

Colonies of extremophiles,

organisms that are living off a very
different environment of gases

to the one that we're used to
on the surface.

They are a window
on a much earlier time.

Because without oxygen,
the ancestors of these extremophiles

were the only forms of life
our planet could support.

(WATER DRIPPING)

Ünderstanding how Earth developed
an atmosphere rich in oxygen

has taken centuries.

And the secret ljes
with ancient bacteria.

In 1676, a Dutchman called
Antoine Leeuwenhoek,

was trying to find out
why pepper İs spicy.

See, they thought that there were
little spikes on peppercorns

that dug into your tongue.

He was using the microscope,

which had been discovered about
50 or 60 years before,

but inexplicably had never been used for
anything useful before.

He put the peppercorns on there

and he looked down and he
couldn't see anything.

So he thought,
"Okay, I'll grind them up,

"dissolve them in water
and have a look."

When he did that, he didn't see anything
interesting in the peppercorns,

but he found that there were
little animals swimming around.

And he said that he estimated
you could line about

a hundred of the "wee little creatures",
those were his words,

up along the length
of the single coarse sand grain.

What Leeuwenhoek thought were animals,

were in all probability
notanimals at all.

Although he didn't know it at the time,

he had discovered a whole
new domain of life.

Bacteria.

They are by far the most
numerous organisms on the EFarth.

In fact, there are more bacteria
on our planet

than there are stars
in the observable universe.

And there is öone kind of bacteria
more numerous than all the rest.

One of the most striking structures
I can see on this slide is a

kind of a blue-green filament which is a

little colony of a type of bacteria
called cyanobacterla.

These things are incredibly
important organisms.

Fossilised cyanobacteria have been found
as far back as 3.5 billion years ago.

And at some point
around 2.4 billion years ago,

they became the first living things
to use pigments to split water apart

and use it to make food.

This evolutionary invention
Was incredibly complex.

Even its name is a mouthful.

Oxygenic photosynthesis.

İt starts with a photon from the Sun
hitting that green pigment, chlorophyll.

Chlorophyll takes that energy

and uses İit to boost electrons
up a hill, if you like.

When they get to the top,

they cascade down a molecular waterfall

and the energy is used
to make something called ATP,

which is essentially the
energy currency of life.

This little molecular machine
is called Photosystem 2,

and it makes energy
for the cell from sunlight.

But when the electrons reach the bottom
of that waterfall,

they enter Photosystem 1.

They meet some more chlorophyll which
is hit by another photon from the Sun.

And that energy
raises the electrons up again

and forces them onto carbon dioxide,

turning that carbon dioxide, eventually,
into sugars, into food for the cell.

Now, why all this complexity?

Why do you need these two photosystems
Joined together in this way

Just to get some electrons and make
sugar and a bit of energy out ofit?

It's because only when life coupled
these two biological machines together,

that it could split water apart
and turn it into food.

But it wasn't easy.

The thing is that
Water is extremely difficult to split.

So for aleaf to doit,
for a blade of grassto doit,

jJust using a trickle of light
from the Sun is extremely difficult.

In fact, the task is so complex

that, unlike flight or vision,

which had evolved separately
many times during our history,

öxygenic photosynthesis
has only evolved once.

Every tree, every plant,
every blade of grass on the planet,

everything that carries out
oxygenic photosynthesis today

does it in exactly the same way.

And the structures inside every leaf
that do that

look remarkably similar
to cyanobacteria.

In other words,
the descendants of one cyanobacterium

that worked out, for some reason,

how to couple of those complex
molecular machines together

in some primordial ocean,
billions of years ago,

are still present on the Earth today.

The cyanobacteria changed the world,

turning it green.

And that had a wonderful conseguence.

With this new way of living,

life released oxygen into the atmosphere
of our planet for the first time.

And in doing so over
hundreds of millions of years,

it eventually completely transformed
the face of our home.

And as the oxygen levels grew,

the stage was set for the arrival
of ever more complex creatures.

But on Farth,
the emergence of complex life

regğuired a rather more
intangible ingredient.

Something that you can't see,
touch or smell.

And yet you pass through every day.

Late January,

and the monarch butterflies
have found their way home.

They've entered a hibernation state,

huddling together for warmth.

But they're only here at all

thanks to one of the most accurate
biological clocks found in nature.

These are the pine and oyamel forests

at high altitude about, what,
three hours north-west of Mexico City

and one of the few wintering grounds of
the monarch butterflies, as you can see.

But there is a colony,
millions of monarchs,

somewhere due north of here.

So if I head off into the forest,

then hopefully this will just be a
taster of what's to come.

To find the butterflies, 1 need to get
an accurate bearing on them.

And to do this, 1 need a timepiece.

If you don't have a compass,

how can you tell which direction's
north and which direction's south?

Well, you can use the Sun.

The Sun rises in the east,
sets in the west.

And at midday in the
northern hemisphere it's due south.

But what ifit isn't midday?

Well, there's an old trick,
which is to use a watch.

See, it's about 3:00
in the afternoon now,

and if you line the hour hand
of your watch with the Sun,

then in the northern hemisphere a line
in between the hour hand and 12:00

will point due south.

Which means that north, is that way.

For thousands of miles
on their way here,

the monarchs have faced
the same problem.

To make their way south,
it's no good simply following the Sun,

because as the day progresses,

the Sun's position drifts
across the sky.

Somehow, they have to correct for this.

They use what's called
a time-compensated Sun compass.

They measure the position
of the Sun every day using their eyes.

But it's also thought they can measure
the position even when it's cloudy,

by using the polarisation of the light.

Having locked onto the Sun,

their brain then corrects
for its movement across sky

by using one of nature's
most accurate timepieces.

By combining the information from their
precise clocks and their eyes,

they can navigate due south.

That ability to orientate themselves is,

Ithink, one of the most
remarkable things I've seen.

The biological clocks
that have brought the monarchs home

are not unigue to butterflies.

Almost all life shares
in these circadian rhythms.

They're an evolutionary conseguence
of living on a spinning rock.

Our world turns on its axis once
every 24 hours, giving us a day.

İt's on a billion-kilometre journey
around the Sun,

and each orbit gives us a year.

We live inside a celestial clock,

öne that has been ticking away
for over four and a half billion years.

And that's a full third
of the age of the universe.

(MEXICAN MUSIC PLAYING)

(HORSE NEIGHS)

This is the final ingredient
that our home has provided.

Time.

Take the horse.

Like all complex living things,

it's here because our planet
has been stable enough

for long enough
to allow evolution time to play.

The horse is the animal whose family
tree we know with the highest precision.

So it's possible to lay out
Just öne unbroken chain of life

that stretches back
nearly four billion years.

Animals that are recognisably horse-like
have been around for a long time.

Something like 55 million years.

You then have to jump duite a lot
to something like 225 million years

if you want to ask the guestion,
where is the earliest mammal?

And it's this thing which looks
something like a little shrew.

535 million.

This is the point when complex life
really began to explode in the oceans.

You then have to sweep back
a long, long time

to find the next evolutionary milestone,

arguabliy the most important milestone.

The emergence of the complex cell,
the eukaryote.

Then, you have to step back
a long way in time.

You have to step back

all the way to here,

the emergence of the prokaryote,
the first life form.

And so we have this beautiful long line.

We can trace my friend, the horse,
and his ancestry back

to events that happened 3.5,
3.6, 3.7 billion years ago

on the primordial Earth.

Now, looking back over
that vast sweep of time

you could ask yourself
the guestion, well,

do you need
three and a half billion years

to go from a simple form oflife
to something as complex as a horse?

Well, the answer to that guestion is
we don't know for sure.

İt seems that you need
vast expanses of time.

But do you need those big gaps from
the simple cell to the complex cell?

Do you need the gap
from the complex cell

to the evolution of the
multi-cellular life?

We don't know.

We only have öne example.

There is only öone planet

where we've been able to study
the evolution of life.

And it's this one.

And Farth has been an interesting
mixture of stability and upheaval.

It's had an environment that's never
completely conspired to wipe out life,

but it's constantly
thrown it challenges.

The deep time that our planet
has given life has allowed it to grow

from a tiny seed of genetic possibility

to the planet-wide web of complexity
we're part of today.

Only a few of us
have ever stepped outside of this world.

But those that have
discovered something rather wonderful.

ASTRONAUT (ON RADIO):
For all the people back on Farth,

the crew of Apollo 8 has a message
that we would like to send to you.

On Christmas Eve, 1968,
my first Christmas Eve,

the Apollo 8 spacecraft entered the
darkness on the far side of the Moon.

ASTRONAUT: Zn the beginning,
God created the heaven and the Farth,

and the Farth was without form.

COX: 7he three astronauts
Borman, Lovell and Anders,

became the first human beings
in history to lose sight of the Farth.

ASTRONAUT: Arıd God salid, "Llet there
be light " and there was light.

And God saw the light, that it was good.

COX: When they emerged
from the dark side of the Moon

and the Earth rose into view,

they chose to broadcast
their culture's creation story

back to the inhabitants of Earth.

And just like the Aztecs and the Mayans
and every civilisation before them,

it told of the origins of their home.

ASTRONAUT: Arrd God called
the dry land Farth,

and the gathering together
of the waters, called He seas.

And God saw that it was good.

İt must be innately human,

the desire to understand how our home
came to be the way thatit is.

Seen from a lunar orbit
against the blackness of space,

the Earth is a fragile world.

But seen by science, it's a world
that's been crafted and shaped by life

over almost four billion years.

So we're on our way to understanding
how we came to be here.

But as the Apollo astronauts discovered,

the journey of discovery has already
delivered much more than just the facts,

because it's given us
a powerful perspective

on the intricacy and beauty of our home.

ASTRONAUT: #rom the crew of Apollo 8,

we close with good night, good luck,
a Merry Christmas,

and God bless all of you.
All of you on the good Farth.